Luminescence and mechanism studies of multi-colored phosphors with rare earth ions or transition metal ion

Abstract

With growing global energy shortage and increasing environmental pressure, energy conservation and environmental protection has become one of the important subjects nowadays. In the field of general lighting, white LED products are attracting more and more attention for their advantages: lightning with high efficiency, long usage life, small size, low power and environmental protection, which is recognized as a new lighting source and would become the future trend of development. And as the most important components in white LEDs, phosphors have significant influence on its performance index: luminous efficiency, service life, color temperature and color rendering index. Generally, phosphors can be doped with rare earth (RE) ions for its special electronic structure, which has strong absorption ability, high conversion rate, strong emitting ability particularly in the visible region and the stable chemical and physical properties. Among them, the most widely used methods to fabricate white LEDs are ultraviolet or near ultraviolet LEDs with tricolor phosphors (red, green, blue) combination, which has the best color and high fluorescent material luminous efficiency; The other is to combine blue LED with the yellow phosphor. In the third chapter, Mn4+-activated cubic phase Ba5AlF13 red phosphors were prepared by the two-step coprecipitation method. The structural and optical features were conducted on the basis of X-ray diffraction (XRD), transmission electron microscopy (TEM), emission and excitation spectra, and luminescence decay curves. The Ba5AlF13:Mn4+ phosphors can be efficiently excited by near-UV to blue light and exhibit the bright red emission at around 627 nm, which is assigned to the 2Eg→4A2g transition of the 3d3 electrons in [MnF6] octahedra. Temperature dependent emission spectra and decay curves from 10 to 550 K were introduced to deep understand the luminescence mechanism of Mn4+ in Ba5AlF13 lattice. Notably, such a novel red phosphor shows excellent anti-thermal quenching behavior (~700% of emission intensity at 300 K relative to 10 K). In the fourth chapter, Ba5Al3F19:Eu2+ blue-emitting phosphors were synthesized by the solid state reaction in a reductive atmosphere. The crystal structure of Ba5Al3F19:Eu2+ phosphors were characterized by using the X-ray powder diffraction (XRD) measurement. The luminescence properties are investigated systematically based on the crystal structure. Emission and excitation spectra of the Ba5Al3F19:Eu2+ phosphors along with the decay curves were characterized. Under the excitation of UV light, the phosphor exhibits a broad band emission around 410 nm ascribed to the allowed 4f65d→4f7(8S7/2) transition, together with a sharp line at 360 nm corresponding to the forbidden 4f7(6P7/2)→4f7(8S7/2) transition of the Eu2+ ions. The temperature dependent emission spectra and decay curves were measured to investigate the origins of the emission and the thermal stabilities of the as-prepared phosphors. The as-prepared Ba5Al3F19:Eu2+ phosphors display interesting and stable luminescence properties, which can act as promising blue-emitting phosphor candidates. In the fifth chapter, Series of Mn4+-activated LiLa2NbO6 red emitting phosphors were prepared by the solid state method. The structural and luminescence properties are investigated on the basis of X-ray diffraction (XRD), emission and excitation spectra, and luminescence decay curves. The LiLa2NbO6:Mn4+ phosphors can be efficiently excited by near-UV to blue light and exhibit bright red emission at around 712 nm, which is assigned to the 2Eg→4A2g transition of the 3d3 electrons in [MnO6] octahedra. Temperature dependent emission spectra and decay curves from 10 to 480 K are analyzed to understand the luminescence mechanism of Mn4+ in LiLa2NbO6 lattice. Notably, such a novel red emitting phosphor show special anti-thermal quenching behavior.